Recently, image pick-up apparatuses such as cameras are being equipped with an automatic focusing function which enables a lens to automatically focus on a subject for the convenience of users. Focus detecting apparatuses detect the focus state and defocus amount of photographing lenses so that lenses can be made to automatically adjust the focus on a subject.
When the chromatic aberration phenomenon occurs, that is, when a refractive index differs depending on the wavelength, errors may occur when detecting the focus state of a photographing lens. Generally, it is desirable to focus on a subject based on a wavelength (about 587 nm) of the green color to which the human eye is most sensitive. However, the wavelengths of the light source(s) illuminating a subject may change according to the surroundings. Focus detecting apparatuses detect the focus state of photographing lenses based on the light incident on focus detecting apparatuses. If the main wavelengths of the light source illuminating the subject significantly deviate from the wavelengths of green, it may be difficult, or even impossible in some cases, for the lens to focus precisely on the subject.
Moreover, in a dark environment, it may be difficult for the focus detecting apparatus to accurately detect the focus state of the photographing lens due to the reduced amount of light available for the focus detecting apparatus. Typically, in such a dark environment, an auxiliary light source, for example, a flash light, is used to provide an additional illumination of the subject. The light provided by such an auxiliary light source typically includes infrared rays. The wavelengths of infrared rays are significantly far apart from the wavelengths of green in the spectrum, also resulting in possible errors when detecting the focus state of the photographing lens.
Studies on focus detecting apparatuses for correcting errors relating to various types of light illumination sources are on-going, but unfortunately thus far without a satisfactory solution for correcting these errors. Additionally, determining the type of light sources may require a number of additional components, likely resulting in the increased manufacturing cost and/or overall size of, and/or in a reduction in the producibility of, an image acquiring apparatus.